"For fixed-effects estimation with cluster, xtivreg2 makes no
degrees-of-freedom adjustment for
the number of fixed effects. This follows the formulation of a
cluster-robust covariance matrix
for the fixed-effects model as originally proposed by Arellano (1987);
see,
e.g., Wooldridge
(2002), p. 275. Stata's official xtivreg, xtreg and areg (as of version
9.1, October 2005), by
contrast, use the (N-N_g-K) adjustment, which is somewhat conservative in
this context. However,
the approach used by xtivreg2 requires that no panel overlaps more than
one
cluster. That is,
either the panel variable is identical to the cluster variable, or panels
are uniquely assigned
to clusters. If any panel is contained in more than one cluster, the
xtivreg2 approach is
invalid, and it will exit with an error."

I found that -xtdata, fe- can be used with -cluster()- without changing
the
degree of freedom, given that the degree of freedom depends only on the
number of cluster!! Besides that, I found that -xtivreg2, small- gives
slightly different standard errors. Is there an additional correction
included in -xtivreg2, fe- that is not in -xtreg, fe-?